A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to display an image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD is often referred to as augmented reality because the viewer's image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”).
HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eyes off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. Due to the infancy of this technology, there is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are currently limited due to the cost, size, field of view, and efficiency of conventional optical systems used to implemented existing HMDs.
FIG. 1A illustrates a conventional near-to-eye optical system 101 using an input lens and two mirrors. Near-to-eye optical system 101 may be used to implement a HMD. An image source 105 outputs an image that is reflected by two conventional flat mirror surfaces 110 and 115, which form an image near to eye 120. Image source 105 is typically mounted above the head or to the side of the head, while mirror surfaces 110 and 115 bend the image around the front of the viewer's face to his eye 120. Since the human eye is typically incapable of focusing on objects placed within a few centimeters, this system requires a lens 125 interposed between the first mirror surface 110 and image source 105. Lens 125 creates a virtual image that is displaced further back from the eye than the actual location of mirror surface 115 by positioning image source 105 inside of the focal point f of lens 125. Optical system 101 suffers from a small field of view (e.g., approximately 20 degrees) limited by the extent of mirror surfaces 110 and 115 and the bulkiness of lens 125.
FIG. 1B illustrates operation of a conventional flat mirror surface 130. As can be seen, each ray (e.g., rays R1, R2, R3) of divergent light emanating from source point S1 is reflected off of mirror surface 130 with a different angle. Although rays R1, R2, and R3 all originate from a common source point S1, each ray is reflected with a different trajectory and ends up at a different destination point D1, D2, and D3. In other words, the reflective properties of mirror surface 130 result in a one-to-one correspondence between divergent rays R1, R2, and R3 from a common source point S1 and destination points D1, D2, D3. Mirror surface 130 cannot reflect two or more divergent rays from common source point S1 to a single destination point. Rather, with flat mirror surface 130 the angles of incidence and departure for each ray are identical and mirrored along the normal vector extending from the surface.